1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method, and particularly, to the apparatus and the method for polishing semiconductor wafers based on a chemical mechanical polishing (CMP) technique. The present invention also relates to a backing plate and a backing film used by the polishing apparatus.
2. Description of the Related Art
FIG. 1A is a top view showing a polishing apparatus according to a related art and FIG. 1B is a side view showing the same. The polishing apparatus is used for semiconductor device manufacturing for polishing and planarizing the steps in the surface of a semiconductor wafer due to devices and interconnections formed thereon. A disk-like surface plate 8 has a shaft 10 rotated by a driver (not shown). A polishing cloth 7 made of for example, polyurethane foam is attached to the top of the surface plate 8. A port 11 supplies abrasive 12 onto the polishing cloth 7. A wafer base 13 is arranged above the surface plate 8. The bottom of the wafer base 13 holds a wafer. The wafer base 13 has a shaft 9, which is connected to a pressing unit (not shown) and a rotating unit (not shown). The pressing unit presses the wafer against the polishing cloth 7. The rotating unit rotates the wafer in the same direction as the rotating direction of the surface plate 8.
FIG. 2 is a sectional view showing the wafer base 13 and the vicinity thereof. The wafer base 13 is made of a head 6, backing plate 4, a backing film 2, and a guide 5. The head 6 is driven by the shaft 9 and rotates above the surface plate 8. The head 6 is pushed down by the pressing unit through the shaft 9. The head 6 uniformly presses the wafer 1 against the polishing cloth 7 through the backing plate 4. To flatly polish the wafer 1, an interface of the backing plate 4 with the backing film 2 is processed flat. The back film 2 is resilient so that the backing plate 4 may evenly press the wafer 1 against the polishing cloth 7. Even if dust is present between the wafer 1 and the backing film 2, the backing film 2 is flexible to contain the dust so that a surface of the wafer 1 to be polished may evenly be pushed against the polishing cloth 7. The guide 5 prevents the wafer 1 from moving away from the backing film 2. An end face of the guide 5 that faces the polishing cloth 7 is higher than the polished surface of the wafer 1 with respect to the polishing cloth 7. When the wafer 1 is set on the polishing cloth 7, the end face of the guide 5 is away from the polishing cloth 7. When the wafer 1 is pressed against the polishing cloth 7, the backing film 2 and polishing cloth 7 are compressed and the end face of the guide 5 comes in contact with and presses the polishing cloth 7.
With this arrangement the port 11 feeds the abrasive 12 onto the polishing cloth 7 that is rotated The wafer 1 set under the backing film 2 is rotated and pushed by the wafer base 13 toward the polishing cloth 7 so that the surface of the wafer 1 contacting with the polishing cloth 7 is polished.
Polishing rates and their uniformity on a thermal oxidation film formed on the surface of an 8-inch silicon wafer will be explained. The wafer has LSIs formed on the surface thereof The size of each LSI is dependent on a step-and-repeat technique used to form the LSIs and is usually 1-cm square. To improve the yield and quality of LSIs on each wafer, polishing rates within the wafer must be as uniform as possible.
FIG. 3 shows polishing rates measured at different measurement points on a wafer. The wafer is a silicon wafer of 200 mm in diameter and has a thermal oxidation film to be polished with the wafer being pressed against the polishing cloth 7 and the guide 5 being away from the polishing cloth 7. The measurement points 1 to 7 are set along a straight line passing through the notch and center of the. wafer and are away from the center of the wafer by 96 mm, 80 mm, 40 mm, 0 mm, 40 mm, 80 mm, and 96 mm, respectively. Namely, the measurement points 1 and 7 are at the periphery of the wafer, and the measurement point 4 is at the center thereof. Polishing rates measured at the points 1 and 7 are each about 1.7 times greater than that measured at the point 4.
FIG. 4 shows polishing rates measured at different measurement points with the guide 5 being pressed against the polishing cloth 7 when polishing a thermal oxidation film formed on a silicon: wafer of 200 mm in diameter. The measurement points 1 to 7 are the same as those of FIG. 3. Polishing rates at the peripheral measurement points 1 and 7 are about 20% smaller than those at the other measurement points. Compared wit FIG. 3, FIG. 4 shows an improvement in the uniformity of polishing rates on the wafer, and therefore, it can be said that pressing the guide 5 against the polishing cloth 7 is advantageous. This, however, may deteriorate the quality of LSIs formed at the periphery of the wafer below criteria because the peripheral polishing rates are about 20% smaller than the others.
The reason why the peripheral polishing rates are lower than the others will be examined.
FIG. 5 is a partly see-trough top view showing essential parts of a polishing apparatus. A polishing cloth 7 is circular, 600 mm in diameter, and about 4 mm in thickness. When polishing a wafer 1, the polishing cloth 7 is rotated counterclockwise at about 30 rpm. A guide 5 is a cylinder having an inner diameter of about 202 mm. When polishing the wafer 1, the guide 5 is rotated counterclockwise at about 30 rpm. The wafer 1 is circular, 200 mm in diameter, and about 0.8 mm in thickness. When being polished, the wafer 1 is pushed against a backing film 2 (not shown) that revolves with the guide 5. At this time, the wafer 1 slightly slides on the backing film 2 and rotates counterclockwise at a speed slower than 30 rpm. Under this situation, the wafer 1 is shifted toward a right part of the ring 5, and a gap 16 of about 2 mm is formed between the guide 5 and the left edge of the wafer 1. In connection with this, two cases will be examined.
(1) A first case is that the guide 5 is away from the polishing cloth 7 even after the wafer 1 is pressed against the polishing cloth 7. FIGS. 6A, 6B, and 6C are sectional views taken along a line Ixe2x80x94I of FIG. 5. In FIG. 6A, the wafer 1 is not pressed against the polishing cloth 7, and the polishing cloth 7 and a wafer base 13 are not rotated yet. A gap 15 between the guide 5 and the polishing cloth 7 is set to be in the range of 0.3 mm to 0.5 mm so that the guide 5 is away from the polishing cloth 7 after the wafer 1 is pressed against the polishing cloth 7 and so that the wafer 1 may not escape from the guide 5 even when the polishing cloth 7 and wafer base 13 are rotated.
In FIG. 6B, a shaft 9 is thrust to press the wafer 1 against the polishing cloth 7. The wafer 1 compresses the polishing cloth 7, and the backing film 2 on the wafer 1 is also compressed evenly.
In FIG. 6C, the polishing cloth 7 and wafer base 13 are rotated from the condition of FIG. 6B. The polishing cloth 7 moves right ward in FIG. 6C, and therefore, the wafer 1 is shifted toward the right part of the guide 5 and the polishing cloth 7 at the edge of the wafer 1 is deformed due to rotation. In particular, the polishing cloth 7 at the left edge of the wafer 1 rises and is compressed further than FIG. 6B. At this time, the left edge of the wafer 1 is pushed more strongly than the remaining part thereof by the polishing cloth 7, to increase a polishing rate at the left edge.
The reason why polishing rates at the periphery of a wafer is about 1.7 times greater than those at the other parts in FIG. 3 is because the periphery of the wafer is pushed more strongly than the other parts by the polishing cloth 7 and because the wafer 1 is rotated by the wafer base 13.
(2) A second case is that the guide 5 is pressed against the polishing cloth 7 when the wafer 1 is pushed to the polishing cloth 7. FIGS. 7A, 7B, and 7C are sectional views taken along the line Ixe2x80x94I of FIG. 5. In FIG. 7A, the wafer 1 is not pressed against the polishing cloth 7, and the polishing cloth 7 and wafer base 13 are not rotated yet. A gap 15 between the guide 5 and the polishing cloth 7 is set to be in the range of 0.21 mm to 0.28 mm so that the guide 5 may be pressed against the polishing cloth 7 when the wafer 1 is pressed against the polishing cloth 7 and so that the wafer 1 may not escape from the guide 5 when the polishing cloth 7 and wafer base 13 are rotated.
In FIG. 7B, the shaft 9 is insist to press the wafer 1 against the polishing cloth 7. The polishing cloth 7 is compressed by the wafer 1, and the backing film 2 on the wafer 1 is also compressed. The guide 5 is pressed against the polishing cloth 7. If the thrust on the shaft 9 is the same as that of FIG. 6B, force of the wafer 1 of pushing the polishing cloth 7 becomes smaller than that of FIG. 6B when the guide 5 is pressed against the polishing cloth 7. The uniformity of the force of the wafer 1 of pushing the polishing cloth 7 of FIG. 7B is substantially the same as that of FIG. 6B because the gap 15 between the guide 5 and the polishing cloth 7 of FIG. 7A is so set. If the gap 15 is narrower, the force of the guide 5 of pressing the polishing cloth 7 will be stronger so that a deformation of the polishing cloth 7 may reach the wafer 1 to deteriorate the uniformity of the force of the wafer 1 of pressing the polishing cloth 7.
In FIG. 7C, the polishing cloth 7 and wafer base 13 are rotated from the state of FIG. 7B. The polishing cloth 7 moves rightward, and therefore, the wafer 1 is shifted toward the right part of the guide 5. At this time, the polishing cloth 7 at the edge of the wafer 1 is deformed by rotation. The polishing cloth 7 at the left edge of a left part of the guide 5 rises and is compressed further than FIG. 7B. On the other hand, the polishing cloth 7 at the right edge of the left part of the guide 5 is stretched. The stretched area of the polishing cloth 7 reaches the wafer 1. Force of the polishing cloth 7 of pushing the wafer 1 in the stretched area is weaker than that in the remaining area, and polishing rates in the stretched area are smaller than the others.
The reason why polishing rates at the periphery of a wafer are about 20% smaller than the others in FIG. 4 is because the force of the polishing cloth 7 of pushing the wafer is small at the periphery of the wafer and because the wafer is rotated by the wafer base 13.
The two cases mentioned above show that force applied to the surface of a wafer is uniform when the wafer is simply pressed against the polishing cloth 7 and becomes uneven when the wafer is rubbed with the polishing cloth 7.
An object of the present invention is to equally distribute force, which is not present when the wafer 1 is pressed against the polishing cloth 7 and is produced when the pressed wafer 1 is rubbed with the polishing cloth 7, over the surface of the wafer 1.
Another object of the present invention is to equalize force on the surface of the wafer 1, thereby equalizing polishing rates on the wafer 1.
Still another object of the present invention is to uniformly polish LSIs formed on a wafer, equalize the quality of the LSIs, and improve the yield of the LSIs.
To accomplish the objects, three ideas are studied:
(1) A first idea is to use harder material for the polishing cloth 7 so that the polishing cloth 7 may not be deformed due to pressing force and friction. If the polishing cloth 7 is hard, it may unevenly contact and polish a wafer. Accordingly, the first idea is inadequate. The polishing cloth 7 must be soft to some extent.
(2) A second idea is that there will be an optimum value for the gap 15 between the guide 5 and the polishing cloth 7 in FIGS. 6A and 7A and that the optimum value will be between the gap 15 of FIG. 6A and that of FIG. 7A. If there is such an optimum gap, force with which the guide 5 pushes the polishing cloth 7 will be smaller than that of FIG. 7C. This force will vary, and therefore, a strongly compressed area of the polishing cloth 7 will move between under the wafer 1 and under the guide 5. This fluctuates polishing rates at the periphery of the wafer 1. Accordingly, the second idea is inadequate.
(3) A third idea is to enlarge the wafer base 13 with respect to the size of the wafer 1 so that the stretched area of the polishing cloth 7 of FIG. 7C may not reach the wafer 1. This, however, produces a highly compressed area not only under the guide 5 but also under the wafer 1. There will be an idea to adjust the edge of the stretched area of the polishing cloth 7 to the edge of the wafer 1. Adjusting the whole edge of the wafer 1 to the edge of the stretched area of the polishing cloth 7 is very difficult. In addition, a play between the guide 5 and the wafer 1 will increase. Accordingly, the third idea is inadequate.
In order to accomplish the objects, the present invention is, first, characterized by that a polishing apparatus for polishing an object, comprises a polishing cloth having a flat bottom face that is moved in a plane containing the bottom face and a top face that is in parallel with the bottom face, a part of the top face being pressed against ba face of the object to be polished and, a guide for surrounding the periphery of the object, the guide being pressed against the top face of the polishing cloth and, a backing structure for pressing an area of the object that is within a predetermined distance from the guide stronger than the remaining area of the object against the polishing cloth. Here, the backing structure consists of a backing plate and a backing film.
A pressing unit thrusts the backing structure to press the object against the polishing cloth. A distribution of force with which the object pushes the polishing cloth is dependent on a distribution of the distance from the guide. The polishing rate is dependent on the force with which the object pushes the polishing cloth. Namely, when the polishing rate is dependent on the distribution of the distance from the guide, changing the distribution of the force equalizes polishing rates on the object. Generally, the guide is pressed against the polishing cloth and the guide and polishing cloth are rotated, force at the periphery of the object of pressing the polishing cloth drops. This force drop is supplemented by the backing structure of the present invention to equalize force of the object on the polishing cloth.
Moreover, preferably, the predetermined distance according to the first feature of the present invention is in the range of 5 mm to 30 mm. The polishing rates on the object become uniform.
Moreover, preferably, the backing structure according to the first feature of the present invention comprises a backing plate that is a flat disk having top and bottom faces that are in parallel with each other and, a cylindrical ring having top and bottom faces that are in parallel with each other and are defined by concentric outer and inner circles, the diameter of the outer circle being equal to the diameter of the backing plate, the center of the top face of the ring agreeing with the center of the bottom face of the backing plate and, a backing film whose hardness is lower than that of the backing plate, having a flat disk shape having the same diameter as the backing plate and top and bottom faces that are in parallel with each other, the center of the top face of the backing film agreeing with the center of the bottom face of the ring.
Since the backing structure has a changing thickness depending on distances from the guide, a distribution of force with which the object pushes the polishing cloth is dependent on a distribution of the thickness of the backing structure. Namely, the distribution of the thickness of the backing structure determines a distribution of polishing rates on the object. Generally, polishing rates on the object are dependent on distances from the guide, and therefore, the changing thickness of the backing structure that are dependent on distances from the guide equalizes polishing rates on the object. The ring provides the same effect as the thickness change of the backing structure. The thickness and width of the ring are determined based on the type of an object to polish. Objects to be polished by the apparatus of the present invention need different abrasives and polishing rates and have different coefficients of friction with respect to the polishing cloth. Therefore, force on the polishing cloth and torque to rotate the polishing cloth depend on an object to polish. Namely, a force drop at the periphery of an object and an area where the force drop occurs are dependent on the object. This is the reason why the thickness and width of the ring must be determined based on the properties of an object to polish.
The present invention is capable of equalizing force, which is not present when a wafer is set on the polishing cloth and is produced when the wafer is rubbed with the polishing cloth, on the surface of the wafer, thereby equalizing polishing rates over the wafer. The present invention is capable of uniformly polishing LSIs formed on a wafer, thereby equaling the quality of the LSIs and improving the yield thereof. The present invention is capable of improving the polishing speed of each wafer, thereby shortening a polishing time of wafers, improving the productivity of LSIs cut from the wafers, and reducing the production cost of the LSIs.
Moreover, preferably, the height of the ring according to the first feature of the present invention is in the range of 30 xcexcm to 60 xcexcm. And the difference between the diameters of the outer and inner circles of the ring is within the range of 10 mm to 60 mm. The polishing rates on the object become uniform.
Moreover, preferably, the backing structure according to the first feature of the present invention comprises a backing plate composed of a disk and a cylindrical ring, the disk having top and bottom faces that are in parallel with each other, the ring having top and bottom faces that are in parallel with each other and are defined by concentric outer and inner circles, the diameter of the outer circle being equal to the diameter of the disk, the center of the top face of the ring agreeing with the center of the bottom face of the disk, and a backing film whose hardness is lower than that of the backing plate, having a flat disk shape having the same diameter as the outer circle of the ring and top and bottom faces that are in parallel with each other, the center of the top face of the backing film agreeing with the center of the bottom face of the ring. Namely, partly changing the thickness of the backing plate provides the effect of partly changing the thickness of the backing structure.
Moreover, preferably, the backing structure according to the first feature of the present invention comprises a backing plate that is a disk having top and bottom faces that are in parallel with each other, and a backing film whose hardness is lower than that of the backing plate, composed of a disk film and a ring film, the disk film having top and bottom faces that are in parallel with each other, the ring film having top and bottom faces that are in parallel with each other and are defined by concentric outer and inner circles, the diameter of the outer circle being equal to the diameter of the disk film, the center of the bottom face of the ring film agreeing with the center of the top face of the disk film, the top face of the ring film being in contact with the backing plate. Namely, partly changing the thickness of the backing film provides the effect of partly changing the thickness of the backing structure. Preferably, the hardness of the ring film according to the first feature of the present invention is equal to that of the disk film. The backing film is capable of monolithically forming.
The second feature of the present invention is characterized by that a backing plate comprises a flat disk having top and bottom faces that are in parallel with each other, and a cylindrical ring having top and bottom faces that are in parallel with each other and are defined by concentric outer and inner circles, the diameter of the outer circle being equal to the diameter of the disk, the center of the top face of the ring agreeing with the center of the bottom face of the disk. When the guide is pressed against the polishing cloth and the guide and polishing cloth are rotated, force at the periphery of the object of pressing the polishing cloth drops. This force drop is supplemented by the backing plate of the present invention to equalize force of the object on the polishing cloth.
Moreover, preferably, the height of the ring according to the second feature of the present invention is in the range of 30 xcexcm to 60 xcexcm. And the difference between the diameters of the outer and inner circles of the ring is within the range of 10 mm to 60 mm. The polishing rates on the object become uniform.
The third feature of the present invention is characterized by that a backing film comprises a flat disk film having top and bottom faces that are in parallel with each other, and a ring film having top and bottom faces that are in parallel with each other and are defined by concentric outer and inner circles, the diameter of the outer circle being equal to the diameter of the disk film, the center of the bottom face of the ring film agreeing with the center of the top face of the disk film. When the guide is pressed against the polishing cloth and the guide and polishing cloth are rotated, force at the periphery of the object of pressing the polishing cloth drops. This force drop is supplemented by the backing film of the present invention to equalize force of the object on the polishing cloth.
Moreover, preferably, the thickness of the ring film according to the third feature of the present invention is in the range of 30 xcexcm to 60 xcexcm. And the difference between the diameters of the outer and inner circles of the ring film is within the range of 10 mm to 60 mm. The polishing rates on the object become uniform.
Moreover, preferably, the hardness of the ring film according to the third feature of the present invention is equal to that of the disk film. The force at the periphery of the object of pressing the polishing cloth easily becomes greater.
The fourth feature of the present invention is characterized by that a method of polishing an object comprises the steps of (a) making the object come in contact with a backing structure and a polishing cloth while a guide being away from the polishing cloth by a gap, and (b) pressing the object with the backing structure and polishing cloth s o that a peripheral area of the object is pressed stronger than the remaining area thereof, and at the same time, pressing the guide against the polishing cloth, and (e) moving the polishing cloth with respect to the object. The guide is properly pressed against the polishing cloth. When the guide and polishing cloth are rotated, a drop of the force at the periphery of the object of pressing the polishing cloth is supplemented by the step of pressing the object of the present invention to equalize force of the object on the polishing cloth.
Moreover, preferably, the gap film according to the fourth feature of the present invention is in the range of 0.07 mm to 0.28 mm, The guide is properly pressed against the polishing cloth.
Moreover, preferably, a boundary between the peripheral area of the object and the remaining area thereof according to the fourth feature of the present invention is away from the guide by a constant distance tat is within the range of 5 mm to 30 mm. When the guide and polishing cloth are rotated, a drop of the force at the only periphery of the object of pressing the polishing cloth is supplemented.
Other and further objects and features of the present invention will become obvious upon an understanding of illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.